The present invention relates to catalysts for the polymerization of olefins CH2xe2x95x90CHR, wherein R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms. In particular, the present invention relates to a catalyst obtained by reacting a solid catalyst component, based on Mg, Ti and halogen, with a particular pair of alkyl-Al compounds. This kind of catalyst is particularly suitable for the preparation of copolymers of ethylene with xcex1-olefins due to its high capacity for incorporating the comonomer while at the same time maintaining high yields.
Accordingly, another object of the present invention is the use of said catalysts in a process for the copolymerization of olefins in order to produce ethylene/xcex1-olefin copolymers.
Linear low-density polyethylene (LLDPE) is one of the most important products in the polyolefin field. Due to its characteristics, it finds application in many sectors and in particular in the field of wrapping and packaging of goods where, for example, the use of stretchable films based on LLDPE constitutes an application of significant commercial importance. LLDPE is commercially produced with liquid phase processes (solution or slurry) or via the more economical gas-phase process. Both processes involve the widespread use of Ziegler Natta MgCl2-supported catalysts that are generally formed by the reaction of a solid catalyst component, in which a titanium compound is supported on a magnesium halide, with an alkylaluminium compound.
In order to be advantageously usable in the preparation of LLDPE, said catalysts are required to show high comonomer incorporation properties and good comonomer distribution suitably coupled with high yields.
The requirement of high comonomer incorporation is particularly important in the case of gas-phase production processes because the use of excessively large amounts of xcex1-olefin in the feed mixture can cause condensation phenomena in the gas-phase reactor. Therefore, the use of a catalyst having a high capacity for incorporating the comonomer would make it possible to lower the amount xcex1-olefin monomer in the feed.
It is known in the art that the use of different co-catalysts can modulate certain;properties of the solid catalyst component like, for example, polymerization activity, ability to produce higher or lower molecular weights polymers, comonomer distribution, etc. In particular, it is reported in the art that the use of dimethylaluminium chloride with respect to a trialkylaluminium, would give catalysts capable of producing ethylene polymers with a broader Molecular Weight Distribution (MWD) and also capable of incorporating a higher amount of comonomer. However, all the above improvements are made redundant by the fact that the yields are dramatically decreased.
International patent application WO 95/17434 discloses a catalyst system aimed at solving this problem. It is characterized by the use of DMAC/trialkylaluminium (TAA) co-catalyst mixtures in molar ratios from 30 to 300. Table 1 of said application shows that when the DMAC/TAA molar ratio is higher than 30, a high Melt Flow Ratio (indicating a broad MWD) and a melt index in the range 10-20 are obtained. The incorporation of a comonomer in this range of DMAC/TAA molar ratio appears to increase slightly as a function of the TAA content (it passes from 2.1% with the use of pure DMAC to 2.3% with the use of a DMAC/TAA molar ratio of 30). The yields however are very low in this range if compared with the TAA alone. In particular, the activity of the best invention example of Table 1 (Example 4) is about 160 times lower than the activity obtained with triethylaluminium (TEAL) alone. On the other hand, said application shows that when DMAC/TAA molar ratios lower than 30 are used, the Molecular Weight of the polymer decreases (the melt index in the range 20-60), the MWD becomes narrower (Melt Flow Ratios lower than 30 are obtained) and, most importantly, at the same time the incorporation of comonomer does not increase (the value of 2.3% remains constant). All the above drawbacks are not offset by the slight increase in activity which, for a DMAC/TEAL molar ratio of 20, remains about 85 times lower than that for TEAL alone.
Contrary to the strong suggestion of using a large excess of DMAC with respect to the alkylaluminium, we have surprisingly discovered that the use of DMAC/alkylaluminum compound co-catalyst mixtures having lower molar ratios gives catalysts with completely unexpected properties. Said catalysts in fact have a very high capacity for incorporating the co-monomer while at the same time displaying activity which is higher than that obtainable by the use of the aluminium alkyl alone.
Accordingly, an object of the present invention is a catalyst system comprising the product of the reaction between (a) a solid catalyst component comprising Mg, Ti, halogen and optionally an electron donor compound, (b) dimethylaluminium chloride (DMAC) and (c) an compound in which the molar ratio between (b) and (c) is lower than 10.
In the reaction with component (a), the DMAC and the alkyaluminium compound are preferably used in molar ratios from 0.01 to 5 and more preferably between 0.3 and 3.
The alkylaluminium compound can be selected from the compounds of formula AlR13xe2x88x92yHy where y is from 0 to 2 and R1 is a hydrocarbon group having from 1 to 15 carbon atoms. Preferably, the alkylaluminium compound (c) is selected from those of the above formula in which y is 0 and R1 is a C2-C10 alkyl radical. Examples of suitable aluminium alkyl compounds are di-(2,4,4-trimethylpentyl)aluminium hydride, triethylaluminum, triisopropylaluminum, triisobutylaluminum, tri-n-hexylaluminum and tri-(2,4,4-trimethylpentyl)aluminium. The use of triethyl- or triisobutylaluminium is especially preferred.
As explained above, the component (a) of the invention is a solid catalyst component comprising Ti, Mg and halogen. In particular, the said catalyst component comprises a titanium compound supported on a magnesium halide. The magnesium halide is preferably MgCl2 in active form, which is widely known from the patent literature as a support for Ziegler-Natta catalysts. Patents U.S. Pat. Nos. 4,298,718 and 4,495,338 were the first to describe the use of these compounds in Ziegler-Natta catalysis. It is known from these patents that the magnesium dihalides in active form used as support or co-support in components of catalysts for the polymerization of olefins are characterized by X-ray spectra in which the most intense diffraction line that appears in the spectrum of the non-active halide is diminished in intensity and is replaced by a halo whose maximum intensity is displaced towards lower angles relative to that of the most intense line.
The preferred titanium compounds are those of formula Ti(OR2)nxe2x88x92yXy, where X is halogen, preferably chlorine, n is the valence of titanium, y is a number between 0 and n, and the R2 groups, which may be identical or different, are hydrocarbon radicals having from 1 to 10 carbon atoms. Particularly preferred titanium compounds are TiCl4, TiCl3, titanium (IV) butoxide and titanium (IV) isopropoxide, trichlorobutoxy titanium (IV) and dichlorobutoxytitanium (III).
The preparation of the solid catalyst component can be carried out according to several methods. According to one of these methods, the product obtained by co-milling the magnesium chloride in an anhydrous state and the titanium compound is treated with halogenated hydrocarbons such as 1,2-dichloroethane, chlorobenzene, dichloromethane, etc. The treatment is carried out for a time between 1 and 4 hours and at a temperature ranging from 40xc2x0 C. to the boiling point of the halogenated hydrocarbon. The product obtained is then generally washed with inert hydrocarbon solvents such as hexane.
According to another method, magnesium dichloride is pre-activated according to well-known methods and then treated with an excess of Ti compound at a temperature of about 80 to 135xc2x0 C. The treatment with the Ti compound is repeated and the solid is washed with hexane in order to eliminate any non-reacted Ti compound.
A further method comprises the reaction between magnesium alkoxides or chloroalkoxides (in particular chloroalkoxides prepared according to U.S. Pat. No. 4,220,554) and an excess of TiCl4 in solution at a temperature of about 80 to 120xc2x0 C.
According to a preferred method, the solid catalyst component can be prepared by reacting a titanium compound of the formula disclosed above with a magnesium chloride derived from an adduct of formula MgCl2.pR3OH, where p is a number between 0.1 and 6, preferably from 2 to 3.5, and R3 is a hydrocarbon radical having 1-18 carbon atoms. The adduct can be suitably prepared in spherical form by mixing alcohol and magnesium chloride in the presence of an inert hydrocarbon which is immiscible with the adduct, operating under stirred conditions at the melting point of the adduct (100-130xc2x0 C.). The emulsion is then quickly quenched, thereby causing the solidification of the adduct in the form of spherical particles. Examples of spherical adducts prepared according to this procedure are described in U.S. Pat. Nos. 4,399,054 and 4,469,648.
The adduct thus obtained can be reacted directly with the Ti compound, preferably TiCl4, or it can be subjected beforehand to controlled thermal dealcoholation (80-130xc2x0 C.) so as to obtain an adduct in which the number of moles of alcohol is generally lower than 3, preferably between 0.1 and 2.5. The reaction with the Ti compound can be carried out by suspending the adduct (optionally dealcoholated) in the liquid Ti compound (generally at 0xc2x0 C.); the mixture is heated to 80-130xc2x0 C. and kept at this temperature for 0.5-2 hours. The treatment with the Ti compound can be carried out one or more times.
The preparation of catalyst components in spherical form according to this procedure is described for example in European Patent Applications EP-A-395083, EP-A-553805 and WO 98/44001. According to a variation of the method described above the preparation of the solid catalyst components can comprise (i) reacting a compound MgCl2.mROH, wherein 0.3 xe2x89xa6m xe2x89xa61.7 and R is an alkyl, cycloalkyl or aryl radical having 1-12 carbon atoms, with a titanium compound of the formula Ti(OR2)nxe2x88x92yXy, given above; (ii) reacting the product obtained from (i) with an Al-alkyl compound and (iii) reacting the product obtained from (ii) with a titanium compound of the formula Ti(ORII)nXyxe2x88x92n, in which n, y, X and RII have the meanings explained above. As mentioned above, the compound MgCl2.mROH can be prepared by thermal dealcoholation of adducts MgCl2.pEtOH, having a higher alcohol content. Preferred titanium compounds used in step (i) and (iii) are titanium tetrahalides, in particular TiCl4 Particularly preferred in step (ii) is the use of the trialkyl aluminum compounds such as those disclosed above.
According to another embodiment, the MgCl2.pR3OH adduct is first thermally dealcoholated according the procedure described above and successively placed in contact with reactive compounds capable of removing the alcohol. Suitable reactive compounds are, for example, alkyl-Al compounds or SiCl4. The adduct thus obtained is then reacted with a titanium compound in order to obtain the final solid catalyst component. The preparation of catalyst components in spherical form according to this procedure is described for example in EP-A-553806, and EP-A-601525.
The solid catalyst components obtained with methods including the use of MgCl2.alcohol adducts show a surface area (by the B.E.T. method) generally of between 20 and 500 m2/g and preferably between 50 and 400 m2/g, and a total porosity (by the B.E.T. method) of higher than 0.2 cm3/g, preferably between 0.2 and 0.6 cm3/g. The porosity (Hg method) due to pores with a radius up to 10.000xc3x85 generally ranges from 0.3 to 1.5 cm3/g, preferably from 0.45 to 1 cm3/g.
In the methods disclosed above the titanium compound to be supported on the magnesium dihalide is normally pre-formed. Alternatively, it can also be produced in-situ, for example by the reaction of a titanium tetrahalide, in particular TiCl4, with an alcohol R2OH or with titanium alkoxides having the formula Ti(OR2)4. When the preparation of the catalyst component includes the use of an MgCl2.pR3OH adduct, the titanium compound can be obtained by the reaction of a titanium tetrahalide, in particular TiCl4, with the OH groups of the residual alcohol present in a combined form in said magnesium dihalide.
According to another embodiment, the final titanium compound can be obtained by the reaction of a titanium tetraalkoxide with halogenating compounds such as, for instance, SiCl4, AlCl3 or chlorosilanes.
In some instances it is convenient that the titanium compound be reduced to a valence of lower than 4. For example, titanium haloalkoxides with a valence of lower than 4 can also be formed by means of the reaction of titanium tetraalkoxides with mixtures of halogenating and reducing compounds like, for example, silicon tetrachloride and polyhydrosiloxanes. Moreover, it is also possible to use a halogenating agent which simultaneously acts as a reducing agent, such as, for instance, an alkyl-Al halide.
As mentioned above, the solid catalyst component to be used in combination with the DMAC/alkylaluminum mixture may comprise an electron donor compound (internal donor), preferably selected from ethers, esters, amines and ketones.
Said compound is necessary when the component is used in the stereoregular (co)polymerization of olefins such as propylene, 1-butene or 4-methyl-1-pentene. In particular, the internal electron donor compound can be suitably selected from the alkyl, cycloalkyl or aryl esters of polycarboxylic acids, such as for example esters of phthalic, succinic and maleic acid, in particular n-butyl phthalate, diisobutyl phthalate, di-n-octyl phthalate and di-n-hexyl phthalate diethyl 2,3-diisopropylsuccinate.
Other electron donor compounds advantageously usable are the 1,3-diethers of the formula: 
wherein RI and RII, which may be identical or different, are alkyl, cycloalkyl, aryl radicals having 1-18 carbon atoms or hydrocarbon radicals that can be linked together to form condensed structures, and RIII and RIV, which may be identical or different, are alkyl radicals having 1-4 carbon atoms.
The electron donor compound is generally present in a molar ratio with respect to the magnesium from 1:4 to 1:20.
As previously explained, the catalysts of the invention are obtained by reacting (a) a solid catalyst component comprising Mg, Ti, halogen and optionally an electron donor compound, with (b) dimethylaluminium chloride (DMAC) and (c) an alkylaluminium compound in which the molar ratio between (b) and (c) is lower than 10.
The reaction between the three components can be carried out in several different ways, depending on which certain properties of the catalysts can be particularly enhanced with respect to the others. On the basis of the following guidelines, the reaction conditions can be properly selected by a person skilled in the art in order to obtain the catalyst having the required balance of properties. For example, a catalyst having a very high activity and a relatively lower capacity for incorporating the co-monomer is obtained by first placing the components (a) and (c) in contact and then reacting the product thus obtained with the component (b). In this case, the component (b) can be added directly to the polymerization reactor. Catalysts having a better a balance between activity and good capacity for incorporating the co-monomer are obtainable by placing the component (a) in contact with a mixture of (b) and (c) or, preferably, by first placing the components (a) and (b) in contact and then reacting the product thus obtained with the component (c). Also in this case, the component (c) or the mixture of (b) and (c) can be added directly to the polymerization reactor. We also found that if the addition of component (c) is in some way delayed, for example because a polymerization diluent and possibly also the monomer are added before, it is possible to obtain a catalyst displaying an exceptional capacity for incorporating the comonomer together with a relatively lower polymerization activity.
In all the above-disclosed methods, the DMAC and alkylaluminum compounds are normally used in solution or suspension in a hydrocarbon medium such as propane, pentane, hexane, heptane, benzene, toluene, or in halogenated hydrocarbons such as dichloromethane, dichloroethane and CCl4.
The component (a) can be used to prepare the catalyst composition as obtained directly from its preparation process. Alternatively, it can be pre-polymerized with ethylene and/or xcex1-olefins before being used in the main polymerization process. This is particularly preferred when the main polymerization process is carried out in the gas phase. In particular, it is especially preferred to pre-polymerize ethylene or mixtures thereof with one or more a-olefins, said mixtures containing up to 20 mol % of xcex1-olefin, forming amounts of polymer from about 0.1 g per gram of solid component up to about 100 g per gram of solid catalyst component. The pre-polymerization step can be carried out at temperatures from 0 to 80xc2x0 C., preferably from 5 to 50xc2x0 C., in the liquid or gas phase. The pre-polymerization step can be performed in-line as a part of a continuous polymerization process or separately in a batch process. The batch pre-polymerization of the catalyst of the invention with ethylene in order to produce an amount of polymer ranging from 0.5 to 20 g per gram of catalyst component is particularly, preferred. The prepolymerized catalyst component can also be subject to a further treatment with a titanium compound before being used in the main polymerization step. In this case the use of TiCl4 is particularly preferred. The reaction with the Ti compound can be carried out by suspending the prepolymerized catalyst component in the liquid Ti compound optionally in mixture with a liquid diluent; the mixture is heated to 60-120xc2x0 C. and kept at this temperature for 0.5-2 hours.
The presence of a pre-polymerization step makes it possible to react the components (a) to (c) of the present invention according to different embodiments. In one of them, the component (a) is prepolymerized by using only an alkylaluminum compound as a cocatalyst. The so obtained prepolymerized catalyst component can then be used in the main polymerization process together with the DMAC/alkylaluminum mixture of the invention thereby obtaining the described advantages with respect to a polymerization step carried out only with the alkylaluminium compound.
According to another embodiment the catalyst component (a) is reacted directly in the prepolymerization step with a mixture of DMAC and alkyaluminum used as co-catalyst. The so obtained prepolymerized catalyst component can then be used in the main polymerization process in combination with a cocatalyst that can be either an alkylaluminiurn compound or a DMAC/alkylaluminium mixture. The use of DMAC/alkylaluminium mixture is preferred. In case an alkylaluminium compound is used as cocatalyst however, the skilled in the art should avoid any washing of the prepolymerized catalyst component in order to preserve its ability to give the advantages described above. As mentioned, the main polymerization process using the catalyst of the invention can be carried out according to known techniques either in liquid or gas phase using, for example, the known technique of the fluidized bed or under conditions wherein the polymer is mechanically stirred. Preferably, the process is carried out in the gas phase.
Examples of gas-phase processes wherein it is possible to use the catalysts of the invention are described in WO 92/21706, U.S. Pat. No. 5,733,987 and WO 93/03078. These processes comprise a pre-contact step of the catalyst components, a pre-polymerization step and a gas phase polymerization step in one or more reactors in a series of fluidized or mechanically stirred bed. The catalysts of the present invention are particularly suitable for preparing linear low density polyethylenes (LLDPE, having a density lower than 0.940 g/cm3) and very-low-density and ultra-low-density polyethylenes (VLDPE and ULDPE, having a density lower than 0.920 g/cm3, to 0.880 g/cm3) consisting of copolymers of ethylene with one or more alpha-olefins having from 3 to 12 carbon atoms, having a mole content of units derived from ethylene of higher than 80%. However, they can also be used to prepare a broad range of polyolefin products including, for example, high density ethylene polymers (HDPE, having a density higher than 0.940 g/cm3), comprising ethylene homopolymers and copolymers of ethylene with alpha-olefins having 3-12 carbon atoms; elastomeric copolymers of ethylene and propylene and elastomeric terpolymers of ethylene and propylene with smaller proportions of a diene having a content by weight of units derived from ethylene of between about 30 and 70%; isotactic polypropylenes and crystalline copolymers of propylene and ethylene and/or other alpha-olefins having a content of units derived from propylene of higher than 85% by weight; impact resistant polymers of propylene obtained by sequential polymerization of propylene and mixtures of propylene with ethylene, containing up to 30% by weight of ethylene; copolymers of propylene and 1-butene having a number of units derived from 1-butene of between 10 and 40% by weight.
The following examples are given in order to further describe the present invention in a non-limiting manner.
The properties are determined according to the following methods:
Melt Index: measured at 190xc2x0 C. according to ASTM D-1238 condition xe2x80x9cExe2x80x9d (load of 2.16 Kg) and xe2x80x9cFxe2x80x9d (load of 21.6 Kg);
The ratio between MI F and MI E (indicated as F/E) is thus defined as the melt flow ratio (MFR).
Fraction soluble in xylene. The solubility in xylene at 25xc2x0 C. was determined according to the following method: About 2.5 g of polymer and 250 ml of o-xylene were placed in a round-bottomed flask provided with cooler and a reflux condenser and kept under nitrogen. The mixture obtained was heated to 135xc2x0 C. and was kept under stirring for about 60 minutes. The final solution was allowed to cool to 25xc2x0 C. under continuous stirring, and was then filtered. The filtrate was then evaporated in a nitrogen flow at 140xc2x0 C. to reach a constant weight. The content of said xylene-soluble fraction is expressed as a percentage of the original 2.5 grams.
Thermal analysis: Calorimetric measurements were performed by using a Mettler DSC differential scanning calorimeter. The instrument was calibrated with indium and tin standards. The weighed sample (5-10 mg), was sealed into aluminium pans, heated to 200xc2x0 C. and kept at that temperature for a time long enough (5 minutes) to allow a complete melting of all the crystallites. Successively, after cooling at 20xc2x0 C./min to xe2x88x9220xc2x0 C., the peak temperature was assumed as crystallization temperature (Tc). After standing for 5 minutes at 0xc2x0 C., the sample was heated to 200xc2x0 C. at a rate of 10xc2x0 C./min. In this second heating run, the peak temperature was assumed as the melting temperature (Tm) and the area as the global melting enthalpy (AH).
1-Butene was determined via Infrared Spectrometry.
The xcex1-olefins higher than 1-butene were determined via 1H NMR analysis. The total area of the 1H NMR spectrum (between 2.5 and 0.5 ppm) was divided in two regions:
A, between 2.5-1.1 ppm for CH2 and CH
B, between 1.1-0.5 ppm for CH3 
The copolymer composition was then calculated using the following equations:             Cn      ⁢              xe2x80x83            ⁢              (                  mol          .                      xe2x80x83                    ⁢          %                )              =                                        I            B                    /          3                Tot            ·      100                  E      ⁢              xe2x80x83            ⁢              (                  mol          .                      xe2x80x83                    ⁢          %                )              =                                        {                                          I                A                            -                              [                                                      (                                                                  I                        B                                            /                      3                                        )                                    ·                                      (                                                                  2                        ⁢                        n                                            -                      3                                        )                                                  ]                                      }                    /          4                Tot            ·      100      
where:
Tot=Cn+E
n=number of 1-olefin C-atoms
IA, IB=integrals of the regions A and B respectively.
Effective density: ASTM-D 1505